Kelvin--Helmholtz Instabilty (ideal gas, smooth interfaces, 3D)
--------------

This is a 3D version of the Kelvin--Helmholtz instability. These initial
conditions are those used to produce the simulations presented by
Sandnes et al. (2025), section 4.3.1.

This test uses particles set up with equal initial spacing and smoothed density
and velocity discontinuities.

Note that the default resolution_eta parameter is consistent with the use of a
Wendland C2 kernel with ~100 neighbours.

Some examples of configuration options with different hydro schemes:

REMIX:
`--with-hydro=remix --with-equation-of-state=planetary --with-kernel=wendland-C2`

"Traditional" SPH (tSPH):
`--with-hydro=planetary --with-equation-of-state=planetary --with-kernel=wendland-C2`